After drilling and casing of an oil or gas well, the well is opened to the surrounding formation for the ingress of oil or gas. The well is opened by perforating the casing and the rock formation beyond the casing using shaped charges. A shaped charge generally comprises a high explosive material located between a case and a liner. A portion of the liner forms a jet which is propelled away from the case when the shaped charge is detonated. The jet is propelled through the casing and into the formation to form a perforation which facilitates the ingress of oil and/or gas.